The present invention relates to a semiconductor laser device, and more particularly relates to a self-sustained pulsation type semiconductor laser device that emits red laser light.
A red-light-emitting semiconductor laser is widely used as a light source for a DVD apparatus. Also, a self-sustained pulsation type semiconductor laser device does not need an external high frequency modulation circuit for reducing external optical feed back noise and therefore can be a key device in terms of size and cost reduction.
Hereinafter, a typical known structure for a self-sustained pulsation type semiconductor laser device will be described with reference to FIG. 8.
FIG. 8 illustrates a cross-sectional structure for a known semiconductor laser device 10. N-type cladding layer 12 made of an n-type AlGaInP layer; active layer 13 with a multiple quantum well structure; first p-type cladding layer 14 made of a p-type AlGaInP layer; saturable absorption layer 15; and second p-type cladding layer 16, which is made of a p-type AlGaInP layer and has a ridge portion, are stacked in this order over an n-type GaAs substrate 11. That is to say, the saturable absorption layer 15 is inserted between the first and second p-type cladding layers 14 and 16.
A current blocking layer 17 of an n-type GaAs layer is formed on the second p-type cladding layer 16 to cover both sides of the ridge portion. A contact layer 18 is formed on part of the ridge portion of the second p-type cladding layer 16, which is sandwiched by the current blocking layer 17. And a cap layer 19 of a p-type GaAs layer is formed on the current blocking and contact layers 17 and 18.
Further, an n-side electrode 20 is formed on the lower surface of the n-type GaAs substrate 11, while a p-side electrode 21 is formed on the upper surface of the cap layer 19.
In order to fabricate the known semiconductor laser device, n-type AlGaInP layer to be the n-type cladding layer 12; active layer 13; p-type AlGaInP layer to be the first cladding layer 14; saturable absorption layer 15; p-type AlGaInP layer to be the second p-type cladding layer 16; and contact layer 18 are stacked in this order over the n-type GaAs substrate 11 by a crystal growth process (i.e., first growth process). Thereafter, the second cladding layer 16 and contact layer 18 are etched and patterned to form a ridge portion. Next, an n-type GaAs layer to be the current blocking layer 17 are selectively formed on both sides of the ridge of the second cladding layer 16 by another crystal growth process (i.e., second growth process). Subsequently, a p-type GaAs layer to be the cap layer 19 is formed on the contact and current blocking layers 18 and 17 by another crystal growth process (i.e., third growth process).
The distribution 22 of laser light, emitted from the active layer 13, is confined in a part of the active layer 13 under the ridge portion. However, self-sustained pulsation is realized because the saturable absorption layer 14 exists within the range in which the light is distributed.
FIG. 9 illustrates the optical output-current characteristic of the known semiconductor laser device. In FIG. 9, curves a, b, c, d and e represent the characteristics of the semiconductor laser device at temperatures of 20xc2x0 C., 30xc2x0 C., 40xc2x0 C., 50xc2x0 C. and 60xc2x0 C., respectively. As can be seen from FIG. 9, non-continuous characteristics resulting form the self-sustained pulsation is observable in the vicinity of the threshold current. It should be noted that the operating current is 86.6 mA when the optical output is 5 mW at room temperature (25xc2x0 C.).
However, in the known semiconductor laser device, the current blocking layer 17 is made of GaAs and therefore absorbs a great deal of red laser light emitted from the active layer 13. For that reason, the internal loss at the optical waveguide is as large as about 20 cmxe2x88x921, thus causing a problem that the operating current of the semiconductor laser device increases.
Furthermore, increased heat is generated from the semiconductor laser device due to the large operating current, and the known semiconductor laser device cannot be built in an optical pickup apparatus for a DVD, which is in the highest demand now. As a result, the known semiconductor laser device is not suitable for practical use.
Moreover, the known self-sustained pulsation type semiconductor laser device needs to perform three crystal growth processes as described above. Accordingly, the device has a problem that it is difficult to cut down the cost required.
In view of the foregoing, it is a first object of the present invention to realize a self-sustained pulsation type semiconductor laser device having a low operating current. It is a second object of the present invention to get the device fabricated by two crystal growth processes.
To achieve these objects, a semiconductor laser device according to the present invention includes: an active layer; a first cladding layer, which is formed on the active layer and is made of (AlX1Ga1xe2x88x92X1)Z1In1xe2x88x92Z1P (where 0xe2x89xa6X1xe2x89xa61 and 0 less than Z1 less than 1) of a first conductivity type; a current blocking layer, which is formed on the first cladding layer and is made of (AlYGa1xe2x88x92Y)Z2In1xe2x88x92Z2P (where 0xe2x89xa6Yxe2x89xa61 and 0 less than Z2 less than 1) of a second conductivity type and has a striped region; and a second cladding layer, which is formed at least in the striped region and is made of (AlX2Ga1xe2x88x92X2)Z3In1xe2x88x92Z3P (where 0xe2x89xa6X1xe2x89xa61 and 0 less than Z3 less than 1) of the first conductivity type. X1, X2 and Y have relationships represented as Y greater than X1 and Y greater than X2. A saturable absorption region for absorbing laser light produced from the active layer is formed in part of the active layer. The part is located under the current blocking layer.
In the semiconductor laser device of the present invention, the aluminum mole fraction (Y) of the current blocking layer is greater than the aluminum mole fraction (X1) of the first cladding layer or the aluminum mole fraction (X2) of the second cladding layer. Therefore, the bandgap energy of each of the first cladding layer, current blocking layer and second cladding layer can be made greater than the energy corresponding to the oscillation wavelength of the laser light produced from the active layer.
Thus, the first cladding layer, current blocking layer and second cladding layer are transparent to the laser light emitted from the active layer, and it is possible to prevent the laser light from being absorbed into the first cladding layer, current blocking layer and second cladding layer, or the current blocking layer among other things. As a result, the semiconductor laser device of the present invention can reduce its operating current.
Also, the current blocking layer is transparent to the laser light, and the distribution of the laser light emitted from the part of the active layer located under the striped region can be easily expanded to other parts of the active layer located under the current blocking layer. Accordingly, the saturable absorption region for absorbing the laser light produced from the active layer can be formed in those parts of the active layer located under the current blocking layer. As a result, the semiconductor laser device of the present invention realizes self-sustained pulsation.
Further, in this structure, the current blocking layer has the striped region and the second cladding layer is formed in the striped region. Accordingly, only two crystal growth processes are needed, and the fabrication cost of the semiconductor laser device can be reduced.
In the semiconductor laser device of the present invention, an effective refractive index difference between the inside and outside of the striped region, which is a difference between first and second effective refractive indices, is preferably equal to or greater than 2xc3x9710xe2x88x923 and equal to or smaller than 5xc3x9710xe2x88x923. The first effective refractive index is determined by a semiconductor multilayer structure existing inside the striped region to vertically sandwich the striped region therebetween and including the second and first cladding layers and the active layer. The second effective refractive index is determined by another semiconductor multilayer structure existing outside of the striped region to vertically sandwich the striped region therebetween and including the current blocking layer, the first cladding layer and the active layer.
In that case, the size of the saturable absorption region formed in the active layer can be moderate, and good self-sustained pulsation is obtainable.
In the semiconductor laser device of the present invention, the active layer preferably has a quantum well structure formed by stacking multiple quantum well layers and barrier layers one upon the other, and a total thickness of the quantum well layers is preferably 0.03 xcexcm or more. In such a case, good self-sustained pulsation can be obtained.
In the semiconductor laser device of the present invention, the first cladding layer preferably has a thickness of 0.10 xcexcm or more and 0.45 xcexcm or less. In such a case, good self-sustained pulsation is obtainable.